There'd be no backsies if it were physical goods. An offer was presented and accepted, agreed payment exchanged (even if free) and goods delivered. The consumer did not cause the pricing error. Voiding it post-sale undermines the equivalency of physical vs. on-line transactions whether it is a game mistakenly sold for free or an unauthorized e-book edition of George Orwell's Nineteen Eighty-Four vs. a sale of two dozen eggs for -$0.02 after coupon. They're using the ongoing service relationship to renegotiate the price post-sale.
To maintain consumer confidence in the system, you swallow the loss and take steps to prevent it from happening again; something like preventing listing if the price is too low (say, under $10?), requiring special authorization for anything below some threshold, re-prompting for the price to be manually entered again twice (like we have to do for both concealed passwords and in-the-clear e-mail addresses), and never accepting a blank field for a price.
Another option: any free or discounted upgrades could require higher payment if the original sale was underpriced. Other app stores manage to track and enforce this (Apple).
That they are even giving a $10 store credit to purchasers is acknowledging it was their error, trying to limit their losses (total royalty payments per sale are more?), but that is not a negotiation. When they gave refunds to people who bought XBOX 360 HD DVD drives, they didn't require returns or documented destruction via firmware bricking of that hardware. (I have not tested whether they also removed support for HD DVD playback from the updated XBOX 360 UI.)
I have no dog in this hunt: I did not make the purchase at issue.